Jea, I hope we will see some more graphics from AAA_3XA too.
For programming: the most things are pretty well done and since it runs in the browser without lag I guess it's also pretty efficient.
What you can try is a bit more randomness by letting the enemies spawn not only left and right, but a bit more in the back- and foreground by adjusting scale by y-coordinate so they appear smaller "the higher"/"in the background" they spawn.
But as I said, I think you are on a good way and we will definitely meet again. Next time without me forgetting to submit xD